Implement new prefix semantics for extension methods

[vsmenon]

See dart-lang/language#671

• Tests / nail down spec change.
• Implement in CFE.
• Implement in Analyzer.

FYI - @lrhn @leafpetersen @johnniwinther @stereotype441 - tentatively marking D27 for discussion.

[dgrove]

This is no longer a P0 for D27, correct?

[leafpetersen]

This is no longer a P0 for D27, correct?

It may be. There's some desire to do the breaking change version of this, in which case I would prefer we land this in D27.

[leafpetersen]

We're moving forward with this for D27. @johnniwinther has a CL implementing the change in resolution for the CFE. I'll work with @stereotype441 to get this done in the analyzer today. I'll take over the CL from @lrhn with language tests. For deferred libraries, we will temporarily disallow importing libraries with extensions as deferred unless all extensions are hidden, and we will relax this shortly (this just gives us breathing room to be sure we have the right affordances).

[vsmenon]

@a-siva - fyi if flutter rolls are starting today.

[leafpetersen]

@bwilkerson will handle the analyzer part.

[a-siva]

Has all of this landed already or is it slated to land today?

[johnniwinther]

CFE support is in the pipeline: https://dart-review.googlesource.com/c/sdk/+/124540

[leafpetersen]

This is done.